Full rank initializers

[MUCDK]

Currently, we don't document that you can also pass a full-rank initializer. Maybe it's redundant, but this is to be discussed.

[MUCDK]

@ArinaDanilina , can we please update the "Linear problems (advanced)" and "Quadratic problems (advanced)" examples with links to the initializers? https://ott-jax.readthedocs.io/en/latest/initializers/linear.html#module-ott.initializers.linear and https://ott-jax.readthedocs.io/en/latest/initializers/quadratic.html?

Thank you!